This invention relates to a semiconductor light emitting device and a semiconductor light emitting unit, and more particularly to a semiconductor light emitting device suitable to applications requiring high current drive such as backlights for liquid crystal displays and high power LED (light emitting diode) lamps, and a semiconductor light emitting unit with this semiconductor light emitting device being mounted.
In recent years, LEDs, among other semiconductor light emitting devices, have shown a remarkable progress in technology, including white light emitting LEDs using ultraviolet excitation of phosphors. Light emission of every display color on the CIE (International Commission on Illumination) coordinates has been made possible. This has further expanded new applications in various LED displays and liquid crystal display backlights, including in-vehicle applications (stop lamp, tail lamp, dashboard, etc.), traffic signals, and portable devices. The demand for device downsizing and high density packaging in these especially new applications requires a surface mount device (SMD) (see, e.g., Japanese Laid-Open Patent Application 2003-60240).
The driving current for conventional SMD semiconductor light emitting devices has an upper limit of about 50 milliamperes because they have high thermal resistance, and thus they can be driven only by low current. Therefore many LEDs must be mounted in applications requiring high power.
On the other hand, in a high current package developed for power transistors, a lead frame with a die-bonded chip is attached to a heat sink with an adhesive having high thermal conductivity, thereby addressing high current operation. However, this structure has room for improvement at the following points:
1. Thermal resistance of the adhesive layer has not been sufficiently reduced.
2. Many assembly steps are involved, which increases assembly cost.
3. The cost rises due to increase in the number of components (heat sink, lead frame, adhesive, etc.).
Additionally, a semiconductor light emitting device with a lead frame having higher thermal conductivity for driving an LED at high current suffers large thermal stress in the solder reflow process. That is, heat for reflow is easier to be transmitted to the LED in the solder reflow process. For this reason, use of a less heat-resistant adhesive such as silver paste for mounting the LED may cause degradation in reliability including degradation in adhesion strength. It is thus desirable to use a highly heat-resistant adhesive such as eutectic solder for mounting an LED. However, conventional SMDs have another problem that it is difficult to mount the LED using eutectic solder because resin for molding the inner lead section of a lead frame has a low heat resistance temperature.